size of steam and exhaust ports

[Kermit]

I have a question for the designers here. I have been reading and found a formula that sounds right but want to run it by everyone here first.

The area of the port in sq in. TIMES the speed of steam in feet per minute

MUST EQUAL

The area of the cylinder in sq in TIMES the speed of the piston in feet per minute


The book suggested that somewhere between 4 and 5 thousand feet per minute of steam was a good target.






Kermit

 

[bearcar1]

Well, I suppose it beats the 'stop cutting when it looks about right' method. But then again, math was NEVER one of my better talents. :-[ Howsoever, that formula does sound rather impressive. :bow:

 

[steamer]

Kermit

Send me an email, I have quite a bit of text book data on that subject.

Dave

 

[Kermit]

Thanks Dave, You'll have some mail in a while.

I found this particular "FORMULA" more my speed. No strange martian writting involved, unless you count the pi thingy for circular mathematics

Here is what I came up with

BORE 1 inch
RADIUS .5 inch
AREA .785 sq in

RUNS@ 400 RPM
TIMES 5 (2.5 inch stroke x 2 directions)
EQUALS 2000 inch per minute Piston Speed
DIV by 12 inches in a foot
EQUALS 166.67 feet per minute Piston Speed

The area of my piston (times) the speed of the piston

Should equal

The area of my port (times) The speed of steam through Port

Given the books suggestion for a speed of 4000 to 5000 feet per minute and knowing that the steams speed is greater the greater the pressure of the steam, I shall aim for a "safe" speed of 2500 feet per minute.

That gives me three of the four parts. SO I should be able to select a proper opening size in square inches. ACCORDING TO THE BOOK. I'm only trying to verify or disprove or otherwise investigate the words found in the book and not giving out design criteria.

Disclaimer being done with; let's crunch them numbers, shall we?

.785 x 166.67= 130.84

Then

130.84 = area of my steam port x 2500 - selected speed of steam

130.84 / 2500 =.05234 sq in

.05234 = 3.141 x r x r

.01667 = r sq

.1291 inches = radius of Port.


Hmmmm. Sounds small but I was thinking of 1/8 holes for the steam(two of them actually). That is what I would have gotten with 5000 instead of 2500 as the Author suggested. THIS MEANS my first guess of two 1/8 inch holes for the steam WAS RIGHT.

Somethings GOTTA BE wrong 8)


Kermit

 

[Philjoe5]

Kermit,
FWIW the #3 mill engine I've built that was designed by Ray HasBrouck has a 1" bore and 0.125" steam ports. Works for me

Cheers,
Phil

 

[steamer]

Hi Kermit,

Sent you a PM.....I can send the reference, but it's a big attachment and can't do it via PM

Send an email if you would.....here's my reference..

A = aLN/c

A = area of steam port

a = area of piston in sq feet
L = length of stroke in feet
N = RPM

c = 6500 feet/min for slow engines......which this engine is

.785 in2 = .785/144 feet2 = .005 feet2

L = 2.5/12 = 0.2083 feet

A = .005 x .2083 x 400 / 6500 = 0.000065 square feet = .009 square inches

Assuming a round port made with a drill, PI D2 /4 = .009 and solving for D

D = .107".............essentially we agree if you account for the different limiting speed......but ...and there is a but

Now 400 rpm is really slow for a such a small engine...if your just putting along at shows that will do just fine, but if your trying to get some real power our of her, you might want to size the ports for say 2000 rpm....where she just might really sing...and you'll get some useful power to boot.....now your talking about .239" diameter...or several smaller holes in line.

Another way to get the port is like this

Machine the port in the block and close the port by pressing in a liner....The port doesn't have to be orthoganal like the drawing and you could mill it in on an angle to shorten the port some.....decreasing the volume of the port which alot of is bad.

Hope that helps!

Dave

 

[rake60]

You all know how I am about keeping it simple. :idea:

Steam (or air) pressure is what makes an engine run.
The bigger the port, the more pressure that port can deliver to a piston.
I don't port size is the limiting factor.

How big of a port can a D-Valve scaled to the engine effectively
switch that port from the intake to exhaust cycle?

OK so I'm just thinking out loud again.....
Or, talking to myself and punching keys on a keyboard.
??? :shrug:

Rick

 

[steamer]

As a PS

I built Rudy's "Little Workhorse" double scale except for the bore which I left .75"

This resulted in a .75 bore x a 1.5" stroke engine with BIG ports...( 4 X stock area) yes she will be a little bit of a steam hog...


Now...put a load on her and run the speed up and I am sure it would remove a finger if given half a chance....

....with a great sounding exhaust "Bark" to boot......

Dave


On Edit, I Cropped the picture

 

[Bernd]

The bigger the port, the more pressure that port can deliver to a piston.

Rick

Ah Rick, don't you mean volume?

Bernd

 

[rake60]

Ah Rick, don't you mean volume?

Bernd

LOL
Na, I mean lots of air!

Rick

 

[steamer]

Your both right! ;D

A large port reduces the pressure loss of the steam ( air) going through the port and results in the Mean Effective pressure rising in the cylinder.....that means more power and higher engine speeds!

Larger clearance volumes on the other hand results in a larger surface for heat loss and condensation....that results in less efficiency of the use of steam....so you shouldn't go "hog wild and pig crazy" with the size of the steam ports....big enough is best..

Dave

 

[Groewrs]

Interesting stuff

I've just been applying steamer's formula to the beam engine I'm currently building. It is a scaled-up version of Bill Lindsay's Beam Engine.

A couple of questions though: The engine I'm building is a Single-acting engine. Does the formula take this into account, or does it not matter? Or am I confusing myself on this one? ??? :-\

I've increased the bore from 3/8" (9.5mm) to 16.3mm (don't ask!) and the stroke from 3/4" (19mm) to 30mm. Bill's drawings show an inlet/exhaust port dia of 1/16". Using Steamer's formulas to calculate the original design RPM gives me a result of about 2900 RPM. Based on this speed, I'm looking at a port dia on my version of around 3.4mm, if I want to achieve the same RPM. Am I on the right track here?

I'm not sure I want the thing to run that quick...

Ah well, interesting discussion anyway! Thanks for bringing it up, Kermit

 

[Kermit]

single Acting Engine!!!

Great Scott, Mr. Methusela. I forgot all about that. ;D Seriously. I was consindering myself "smart" for remembering that the stroke is only half the length traveled in a full revolution. I should have stayed on that train of thought and realized the steam only enters for half the time.

However! Having now boarded and traveled said train of thought. It occurs to me that the formula asked for a SPEED for the piston and for the steam.

:bow:

;D During the first reading of your post to the completion of my reply,I've gone from thinking - "Oh-what a dummy I am" to " The brain isn't completely mush yet! "

Thanks for riding my train.... I think 8)

Edit: Only by making the opening smaller than the formula value would you (according to the formula) have any negative or limiting effect on the engine. Going larger would definately increase leakage in this type of valving. FOR sure.

 

[Groewrs]

I think we are on the same Train, Kermit

Thanks for your help. I think I'll just do it the old fashioned way - Start small, at about 2mm, and test run from there. I can always drill the passages out a bit, but it is a lot harded to make them smaller again ;D3

Thanks

 

[Kermit]

.1291 inches = radius of Port.

Hmmmm. Sounds small but I was thinking of 1/8 holes for the steam(two of them actually). That is what I would have gotten with 5000 instead of 2500 as the Author suggested. THIS MEANS my first guess of two 1/8 inch holes for the steam WAS RIGHT.

Somethings GOTTA BE wrong 8)

Yep! I was right. It's not 1/8 inch wide hole. it's 1/8 inch RADIUS. The hole should be 1/4 inch!!!

Damn... things were looking much more Correct until I found my error. But HEY - I found the error! NOW I feel much more relaxed about my calculations. I'm never right first time around.

 

[steamer]

A couple of questions though: The engine I'm building is a Single-acting engine. Does the formula take this into account, or does it not matter? Or am I confusing myself on this one? ??? :-\

The double acting exhausts out a differant passage....so with a wobbler, you should probably compromise the size between the intake and exhaust. My referace states the exhaust should be 1.5 times the steam port.......so some where in between...1.25?

Dave



A 2900 rpm wobbler is really wobbling!......does a wobbler need to go that fast?...there's a lot of metal flying around.

On the other hand my compound sounds impressive with 180 psia air on it and no load.......fun to do!..but I had better be sitting on it when I do it! ;D

Dave

 

[bearcar1]

Kermit, let me see if I understand this correctly. If I follow your calculations, and they seem to be accurate, I have no reason to believe that they are not, coupled with the fact that I am not smart enough to know any different, the port(s) on a steam engine cylinder of 1.000" bore and 2.5000" stroke should optimally have a an are of two .125" dia. holes? Or a single .250" dia. port? The reason that I am asking is that the engine currently being followed in the 'build in progress' thread calls for a single .125" port at each end. Of course I am not planning to take this engine up to 2500RPM and it only has a 1.800" stroke but you have me wondering if this is substantial enough for my requirements. I'm sure that it is but now is the time to make changes if not. Thank you from a mathematically brain dead cretin. ;D

BC1

 

[steamer]

Bearcar1,

I think again, it gets back to speed.

Depends on the application. For a slow moving engine run lightly, it's not that important.

Run the speed up or go looking for lots of power.....port sizes matter.

Dave